1. Technical Field
The present invention relates in general to rechargeable batteries and, in particular to a method of controlling charge and discharge of a pair of rechargeable batteries utilized as individual power supplies in an electronic apparatus.
2. Description of the Related Art
Because of increase in demand for mobile computing, portable personal computers including notebook-type personal computers (PC), subnotebook-type PCs, palm-top-type PCs, and Personal Data Assistants (PDA)(hereafter collectively referred to as portable PCs) have been developed.
A typical portable PC has a built-in battery, which allows the user to use the portable PC in an environment, such as the inside of a train, in which a commercial power supply cannot be used. The built-in battery is typically a rechargeable battery that can be repeatedly discharged and charged.
In an environment in which a commercial power supply is available, a user connects an AC adapter (i.e., unit for receiving commercial alternating current (AC) and converting it to direct current (DC) for use by a portable PC). Use of the AC adapter makes it possible to charge a rechargeable battery of the portable PC while the portable PC is being powered by the converted DC current.
A rechargeable battery has limited capacity and therefore is only able to power a portable PC for a limited time. To increase the battery powering time for portable-PC, portable PCs are often equipped with two rechargeable batteries, which may be built in. These two rechargeable batteries are referred to as a main battery and an auxiliary battery. The portable PC initially operates by using the auxiliary battery as a power supply. Then, when the capacity of the auxiliary battery is exhausted, the portable PC switches from the auxiliary battery to the main battery to continue powering.
Typically, a charger is capable of charging only one rechargeable battery at a time (i.e., the charger generates only enough charge to fulfill the capacity requirements of a single battery). Therefore, to charge both the main and auxiliary rechargeable batteries, the main battery is first charged followed sequentially by the auxiliary battery. However, because the two batteries are charged in sequence, the time required to charge both batteries is the sum of the two individual times. If both batteries have similar capacity, then the time to charge both batteries is double the time to charge one battery.
Japanese Published Unexamined Patent Application No. 9-103033 discloses a method by which a main battery up to the 50% capacity by one charging circuit, then charges an auxiliary battery up to the 50% capacity, and thereafter connects the main battery and the auxiliary battery in parallel to charge both batteries at the same time, thereby decreasing the charging time.
As described above, the portable PC uses a main battery after completely discharging the auxiliary battery. The effect decreases as a main battery is used less. When a main battery is not used, however, the method fails to show the effect. Normally, when a portable PC is used, the auxiliary battery is not frequently completely discharged because charging is performed early. Therefore, the main battery is rarely used. The method disclosed in the above application is thus not always effective at the time of considering an actual operating state of a main battery or auxiliary battery. Thus, the problem of decreasing the time for charging a plurality of rechargeable batteries still remains unsolved. The present invention recognizes and solves the above problem.
It is an object of the present invention to provide a method of controlling charge and discharge of a plurality of batteries, by which the plurality of batteries may be efficiently charged in a short time.
The foregoing object is achieved as follows. Disclosed is a method for controlling efficient charge and discharge of a plurality of batteries. At the beginning of the discharge cycle, parallel discharge of the rechargeable batteries is performed until the batteries discharge to a predetermined percentage of total capacity. Thereafter, the batteries are discharged serially with the secondary battery being fully discharged before the primary battery is discharged. Thus, the rechargeable batteries each have are at least partially discharged at the start of charging. When charging commences, a serial charge of the rechargeable batteries is first completed until predetermined percentages of capacity are realized. The serial charging is completed with full constant current. Following, the batteries are charged in parallel utilizing a constant voltage such that the current decreases as the charges approach 100%. As a result, the plurality of batteries are charged more efficiently and in a shorter time than if charge individually or serially.
In addition, a method is disclosed of controlling charge and discharge of a plurality of rechargeable batteries in an environment where the battery charger does not provide sufficient capacity to parallel-charge the rechargeable batteries. When a rechargeable battery is being charged from a completely discharged state up to a fully charged state, the charging efficiency (that is, the charging current over the charging cycle) is high at the beginning of the charging cycle and gradually decreases when approaching the end of the charging cycle. The charging current also decreases as the charging time passes and subsequently the margin of the charger increases. Based on these characteristics and other factors, the invention switches from sequential to parallel charging. The present invention makes it possible to control a discharging and recharging sequence of a plurality of rechargeable batteries.
All objects, features, and advantages of the present invention will become apparent in the following detailed written description.